NetMath tools
19 tools that run in your browser. Nothing to install, nothing uploaded.
Network maths that has to be exact: subnetting, CIDR ranges, address conversion and the reference tables engineers reach for. These are pure calculations that run entirely in your browser — no lookups, no logging — so a subnet result is the same whether you check it here or work it out by hand. Where a calculation has a well-known trap (a /31 point-to-point link, a non-contiguous ACL mask, a value that overflows signed 32-bit arithmetic), these tools handle it correctly rather than producing a confident wrong answer.
- Bandwidth-Delay Product / TCP WindowFind how much data must be in flight to keep a long link busy.
- CIDR to IP RangeExpand any CIDR block into its network, broadcast and usable host range.
- HTTP Status Code ReferenceLook up any HTTP status code — what it means and when you actually see it.
- IP Range to CIDRTurn a start and end address into the fewest CIDR blocks that cover it.
- IP to Decimal / Hex / BinaryConvert an IPv4 address to its integer, hex and binary forms, and back.
- IPv4 to IPv6 MapperConvert an IPv4 address to its IPv6 representations.
- IPv6 Expand / CompressExpand an IPv6 address to all eight groups, or compress it back correctly.
- MAC Format ConverterConvert a MAC address between the four notations, and read the two bits that matter.
- MAC Vendor (OUI) LookupFormat any MAC address and read the properties encoded in it.
- MTU / MSS & Fragmentation CalculatorWork out the real MSS once tunnels and headers have taken their cut.
- Ping to Distance CalculatorConvert a round-trip time into the furthest that server can possibly be.
- Port Number ReferenceLook up what service uses a port — and which ports should never face the internet.
- Private / Reserved IP CheckerCheck whether an IPv4 address is private, reserved, or routable on the internet.
- Punycode / IDN ConverterConvert a domain to and from Punycode, and see whether it could be imitating another.
- IP Subnet CalculatorAddress plus mask into network, broadcast, usable range and host count.
- Subnet Mask to CIDRConvert between prefix length and subnet mask, with host counts.
- URL Parser & EncoderPercent-encode or decode a URL, and see the parts it breaks into.
- VLSM Subnet PlannerFit a list of subnets into one block without wasting address space.
- Wildcard Mask CalculatorConvert between a subnet mask and the wildcard mask Cisco ACLs use.
Arithmetic that has to be exactly right
Everything in this category is pure calculation on numbers you supply — no lookups, no network requests, no logging. That is not only a privacy point; it is why the answers are deterministic. A subnet calculation has one correct result, and the same input produces it here, on a whiteboard, or in the router config. What separates a trustworthy implementation from a careless one is the edge cases: a /31 point-to-point link with no broadcast address, a /32 host route, address arithmetic that overflows signed 32-bit maths somewhere above 2.1 billion, and wildcard masks whose bits need not be contiguous. These handle those rather than producing a confident wrong answer.
The mistakes that cause real outages
Three errors account for most subnetting incidents. The first is assuming usable hosts equals two to the power of the host bits — you lose the network and broadcast addresses, so a /24 offers 254 hosts and not 256. The second is confusing a subnet mask with a Cisco wildcard mask in an access list: they are bitwise inverses, and 255.255.255.0 where you meant 0.0.0.255 silently matches the wrong traffic instead of failing loudly. The third is planning adjacent subnets that overlap because the block was not aligned to its own size — a /26 has to start at .0, .64, .128 or .192, and any other starting address is not a valid block however tidy it looks.
Reference tables worth having open
Alongside the calculators, this category holds the lookups that come up mid-diagnosis: what a given HTTP status code actually means and when you genuinely see it, which service owns a port number and which ports should never be reachable from the internet, and how to read the properties encoded into a MAC address — the vendor prefix, whether the address is locally administered, and whether it is a multicast address. These are the details it is faster to check than to half-remember, and half-remembering them is how a firewall rule ends up open to the world.
Frequently asked questions
Why does a /24 have 254 usable hosts and not 256?
The first address in any block is the network address and the last is the broadcast address, and neither can be assigned to a host. So 2^8 addresses give 254 usable ones. The exceptions are worth knowing: a /31 is defined for point-to-point links and uses both of its two addresses, and a /32 is a single host route with no usable range at all.
What is the difference between a subnet mask and a wildcard mask?
They are bitwise inverses. A subnet mask such as 255.255.255.0 marks the network bits with ones; the matching wildcard mask, 0.0.0.255, marks the bits to ignore. Cisco access lists use wildcard masks, and entering a subnet mask where a wildcard belongs does not throw an error — it silently matches the wrong traffic, which is why the converter exists.
Is anything I enter here sent to a server?
No. Every tool in this category is pure arithmetic performed in your browser, with no lookup step at all. That is why the results are identical to what you would work out by hand, and why you can use them on internal addressing plans without thinking about whether a third party is logging your network layout.
Can I subnet an IPv6 network with these?
The conversion tools handle IPv4-to-IPv6 representation, but the subnet calculators are IPv4. IPv6 subnetting is a genuinely different exercise — the address space is large enough that the host-count arithmetic driving IPv4 planning stops being the constraint, and the standard practice is to allocate on nibble boundaries and give every link a /64 regardless of how few devices sit on it.